The invention has been developed in connection with the manufacture of barrels for downhole pumps, such as are used in oil wells. Thus, because of this history, the invention will be described in connection with pump barrels, and the problems associated therewith. However, it is to be understood that the invention will also extend to the manufacture of long, thin-walled, small inside diameter tubes utilized in other applications.
By way of background, a downhole pump consists essentially of a steel barrel containing a reciprocating plunger, for moving the fluid, and suitable valve means, for controlling the admittance and discharge of fluid into and out of the barrel. The barrel is long, typically having a length in the range 6-30 feet. It is of small internal diameter, typically falling in the range 11/4"-31/4". And it is thin-walled, typically having a tube wall thickness in the range 1/8"-1/4". In its simplest form, the barrel comprises a straight tube having strict tolerances with respect to internal diameter and concentricity of the internal diameter relative to the longitudinal axis of the barrel. Since efficient pumping requires minimal clearances between the plunger and the barrel inner surface, internal wear of the barrel wall is the prime determinant of barrel life, particularly, as so often happens, when abrasive particles are present in the pump fluids.
Initially, these barrels were constructed from soft metal pipe, but wear on the barrel rapidly resulted in loss of tolerance limits. In order to increase the service life of the barrels and, to provide abrasion resistance therefor, it was found necessary to harden the internal wear surface thereof.
Historically, the earliest hardened barrels were produced utilizing conventional heat treatment techniques. The barrel would be through-heated to the austenitic temperature in a furnace and then quenched by dropping it into a body of water. However, whilst the barrels so formed exhibited increased wear resistance, these through-hardened barrels lacked the requisite toughness or impact strength properties, and, additionally, were found to be undesirably prone to stress corrosion cracking.
Attempts were then made to harden the inner surface only, utilizing an internal flame followed by a quench ring. The problem with this process resided in the inability to limit the heating to the inside layer of the thin wall, and thus prevent austenite being generated deeper in the wall than was desirable. Stated otherwise, the system did not permit enough heat to be applied in a short enough time and the quench coolant to be applied soon enough thereafter in sufficient volume, so that only a thin internal surface layer was heated to be austenitic temperature and that layer was cooled sufficiently quickly to form a sufficiently hard martensitic microstructure.
It has also become known to harden carburized or carbonitrided tubing by induction heating of the outside diameter of the barrel, or by conventional heating followed by a water quenching step. The inner surface only of the starting tube would have a high carbon content. Thus hardening would occur only at the inner surface section, while the thick outer section would remain tough and ductile, as is desired. The major disadvantages of this technique are associated with the lengthiness and high cost of the carburizing stage. Also, because of the high cost of carburizing, there is a tendency to shorten the carburizing treatment. This results in a very thin case being produced. During the subsequent honing operation, this case is frequently found to have been completely, or partially, removed.
A recent development in the barrel hardening field employs steel tubing having sufficient carbon content to harden without provision of selective surface enrichment with either carbon or nitrogen. This process utilizes low frequency induction heating, typically 3 kHz to 10 kHz, to through-heat the entire barrel wall from the outside inwardly. The tube, upon attaining the requisite quenching temperature, is quenched on the inside only, to thereby form an internal hardened case. In some cases, an external quench may also be applied. In this process, a short coil is positioned on the outside wall of the tube. The coil is functional to traverse the barrel, heating limited areas at a time, to thus limit the power requirements. The problem with this process is that the outer section of the barrel wall becomes hardened and is in tension. This condition is particularly susceptible to hydrogen embrittlement and cracking.
A search of the prior art has been conducted. Several patents, exemplary of which are U.S. Pat. Nos. 2,556,236 to Strickland, and 2,547,053 to Somes et al, were located which disclosed internal electromagnetic induction heating, followed by quenching, to thereby harden the internal bores of short workpieces, such as cylinders, bushings or the like.
The prior art patents specified above disclosed broadly the metallurgical concepts of:
rapidly heating a rotating, translating, tubular workpiece by means of an internally positioned, closely conforming magnetic induction coil, to thereby raise the temperature of a thin surface layer to a value greater than the austenitizing temperature, without heating the relatively thick outer portion of the wall to the austenitic temperature; and
immediately quenching the austenitic surface layer to produce a tube having a relatively thin, hardened, martensitic inner surface layer, of case, having a Rockwell C hardness (HRC) in the order of 58-61 and exhibiting a compressive residual stress pattern, said case being contained by a relatively thick outer wall section which is in a tough, substantially non-hardened condition.
Mechanically, these prior art patents disclosed the following assemblies:
the provision of coaxial tubular power conductors extending along the axis of the tube from its first end and being connected with an induction coil for heating, said conductors being separated by insulation and otherwise being contiguous, said inner conductor forming a bore for supplying cooling water to the hollow coil;
the provision of a quench ring or head, secured to the end of the coil and connected to a water supply mandrel extending along the axis of the tube from its second end, said quench head having a spray outlet directed toward the tube's second end, so that the cooling water discharges away from the coil;
the bore of the coil communicating with an outlet so that the coil cooling water is discharged toward the second end of the tube;
said workpiece, coil, quench head, conductors and quench water mandrel being vertically disposed.
The prior art exemplified by these patents further recognized that it was necessary:
to supply high frequency, high density power rapidly to the workpiece, to obtain quench-hardening temperature of the thin surface layer and ensure non-hardening of the outer portion of the wall; and
immediately quenching to thereby successfully harden the thin surface layer.
However, it is to be noted, that the above-mentioned patents employed short tubes having large internal diameters, typically about 6.5 inches. Additionally, the composition of the workpieces comprised alloy steels, which steels are relatively easily hardenable, entailing less criticality in the hardening process thereof with regard to the heating and quenching parameters.
There are, however, serious problems which arise when one attempts to apply these prior art techniques to elongated, thin-walled, small diameter tubes of a plain carbon steel or non-alloy containing feedstock, such as those used to form downhole pump barrels. These problems have presumably heretofore prevented the application of the internal induction heating technology, in a commercially viable process, to such barrels.
More particularly, the problems to be addressed included:
transmitting an adequate quantum of ratio frequency power to the coil, given the long length of the conductor and thus the attendant line losses;
developing adequate electromagnetic coupling between the internally disposed induction coil and the workpiece, to thereby enable the necessarily large amount of power to be drawn from the generator;
developing high surface power density in the workpiece layer to be hardened, thereby supplying sufficient heat at a sufficiently rapid rate to the layer to attain a temperature therein substantially exceeding the austenitizing temperature and to thus effect the necessary carbon dissolution and homogenization in the metal before significant through wall heat transfer can result;
preventing back-flow of the quench water into the induction heating zone, which back-flow would counteract the heating effect;
delivering a sufficient volume of quench water at a suitable temperature to the heated austenitic surface layer, instantly upon termination of heating, to produce a case having martensitic microstructure;
overcoming the mechanical limitations imposed by the small internal diameter of the tube and the warpage problem associated with the thin-walled nature of the long tube;
and performing the above on the commercially available grades of tubing with normal manufacturing dimensional tolerances.
The ideal characteristics for a pump barrel comprise a tube formed of a composite material having a tough and ductile microstructure in the bulk of the wall thickness and a hardened internal surface layer or case. These properties impart abrasion resistance to the inner wear surface and impact resistance to the outer surface, together with a minimization in propensity to stress corrosion cracking. Preferably, the hardened case should have a thickness of less than 1.0 mm, a hardness exceeding HRC 58, and a substantially uniform martensitic microstructure. Furthermore, the barrel should exhibit a favorable residual stress distribution, with the inside surface preferably being in the highly compressive condition, and the outer core being non-hardened and exhibiting a low tensile residual stress condition. Typically, a desirable hardness profile would demonstrate a sharp demarcation between the hardened and non-heat affected zones.
The ideal characteristics for a manufacturing process to produce such a pump barrel would include relatively high treating speed and low cost.
It is an objective of the present invention to provide a solution to the afore-mentioned problems and further to provide a long, thin-walled, small-diameter tube having the preferred physical and metallurgical characteristics for a pump barrel.